Explosively actuated tools for driving a fastener such as a pin into a substrate such as concrete or steel conventionally comprise a driving piston which is driven forwardly along the barrel of the tool upon detonation of an explosive charge to drive into the substrate a fastener within the forward end of the barrel. After the firing stroke has been to completed, the driving piston is within the forward end of the barrel and appropriate action must be taken to reset the piston into a rear position within the barrel in preparation for the next driving stroke. This may be achieved by a manual action by the operator. One method of manually resetting the piston and which is widely used in practice involves the operator drawing the barrel forwardly from the housing of the tool while the piston is restrained so that the piston lies within a rear part of the barrel which is then retracted manually back into the housing.
There have been proposals for automatic or semi-automatic resetting of the piston. One such proposal involves the use of the explosive gas generated on firing the tool to drive the piston back into its rear position within the barrel after firing. Such a system can however lead to safety problems as the ducting of the explosive gas to a piston return mechanism can result in accumulation of unburnt explosive powder within the mechanism. Further, the problem arising from unburnt residues may be compounded if the tool is not used shortly after resetting of the piston and is subject to rough handling or vibration, for example by being transported on the floor of a truck, whereby the previously reset piston can move out of its predetermined rear position thereby leading to loss of power and possible generation of increasing amounts of unburnt residue at the next firing action.
Alternative proposals for an automatic piston return mechanism involve the use of spring energy which is stored during the driving stroke of the piston and is then released at the end of that stroke to return the piston to its rear position within the barrel. This system however suffers from the disadvantage that part of the driving force of the piston is used to provide the energy for return or resetting of the piston whereby the effective power of the tool is reduced.
In a conventional explosively actuated tool of the type under consideration, firing of the tool generates a recoil effect similar to that experienced upon discharge of a firearm such as a pistol of rifle and this recoil effect can be quite tiring to the operator of the tool.